This invention relates to a new drawing nozzle for the division of melts according to the nozzle blasting process, and it also relates to a process for the division of melts using the drawing nozzle.
The nozzle blasting process is a very old process for separating mineral melts into fibers. It is distinguished in particular, by its simplicity, there being no mechanically activated parts which come into contact with the hot mineral melt. Nevertheless, the nozzle blasting process has not been able to replace two-stage processes, because the fibers which are obtained by the nozzle blasting process are not long enough and have an unsatisfactory fiber thickness distribution with a relatively large average fiber thickness. Therefore, to obtain longer fibers, processes which comprise a first mechanical fiber formation stage employing centrifugal forces (spinner wheel or spinner basket process), followed by a second aerodynamic drawingout stage, have proved successful.
For the production of thin fibers having a diameter of less than 5.mu., industrial two-stage aerodynamic processes are used, the first stage comprising a nozzle blasting process and the second stage comprising a blasting process (Jet-Blast).
It was only the finding disclosed in European Offenlegungsschrift No. 38,989, that a two-stage fiber formation may be carried out in the nozzle blasting process by a specific design of the drawing nozzle, which led to the development of a competitive nozzle blasting process. According thereto, the flow of melt entering the drawing nozzle is split up into a plurality of individual threads (stage one) due to a suitably developed drop in pressure dp/dl in the inlet to the drawing nozzle, and the individual threads are then further drawn out (stage two) in an adjoining drawing-out zone under a substantially constant pressure. Particularly critical parameters of this process include the inlet flow, the transition from the inlet flow to the drawing-out flow and the maintenance of a substantially constant pressure over an adequate length in the drawing-out zone.
In order to take these critical parameters into account, according to the specific solutions proposed in European Offenlegungsschrift No. 38,989, the pressure above the drawing nozzle inlet and the pressure at the end of the drawing-out region are predetermined. When propulsion jet nozzles are preferably used to produce the drop in pressure, these nozzles are positioned below the drawing-out region.
However, it would be desirable for reasons of energy and other reasons, to position the propulsion jets above the drawing-out region. In an arrangement of this type, the propellant gas would also be available as a cooling medium in the inlet region of the drawing nozzle (cooling the upper edge of the nozzle) and in the drawing-out region, and it would also be available as a drawing-out medium, for diluting the fiber/gas dispersion, and for cooling the nozzle wall. This was opposed by the fact that the mixing of the propulsion jets with the air which was drawn into the nozzle inlet by suction, hereinafter termed suction air, constitutes a major disruption to the fiber formation process.